Bent glass plates are used for, for example, building windows, handrails, partitions and show windows as well as vehicle windshields, door glasses, side view mirrors, and so on. In recent years, bent glass plates have been used for more advanced applications. For example, in a head-up display for vehicles, a concave mirror made of a bent glass plate coated with a reflective film is used as an optical component for magnifying and projecting liquid crystal display images onto a windshield. Furthermore, bent glass plates are used as cover glass in some smartphones.
Compared to glass plates conventionally used as glass panes for buildings or vehicles, bent glass plates for head-up display concave mirrors and smartphone cover glass, among others, are small in dimension, but their requirements are far more demanding in terms of for example, accuracy in size, thickness and shape as well as smoothness and optical homogeneity. This is due to the needs to display accurate and high-precision images; visually recognizable images must contain a large amount of information with high resolution. Thus, any method of fabricating bent glass plates for use for smartphone cover glass or head-up display concave mirrors must meet more strict precision standards than those for conventional applications.
Methods of press-molding a glass member may be generally classified into two groups. One method is the “direct press method,” in which raw glass material is melted, defoamed and homogenized; while being still hot, the molten glass flowing out of a circular tube feeder is cut and divided into chunks, each having a predetermined volume, and each of the chunks is pressed with dies to form an intended shape. The other method is the “reheat press method,” in which a glass material is precut into a predetermined volume in order to prevent the glass material from having more than required surface area relative to its volume; then, the cut glass material is reheated and softened to a predetermined viscosity to be press-molded into its intended shape.
In the direct press method, it is possible to utilize the heat added to melt the raw glass material to also press-mold the material; therefore, this method has a characteristic of offering high energy efficiency. In this method, however, the molten glass during the pressing operation needs to be kept at a high temperature, because of the reasons that the molten glass needs to be cut into chunks each having a predetermined volume while being hot, and the molten glass needs to reach a suitable viscosity for flowing out of the feeder with a predetermined flow rate (the suitable viscosity is generally lower than an optimal viscosity for shaping), among other reasons. As a result, it becomes difficult to produce molded articles with high precision due to high thermal contraction of the glass member while cooling down to ambient temperature after the press molding. Therefore, the direct press method is unsuited for fabricating a bent glass plate with high-precision shape.
The reheat press molding of a glass member is a method comprising steps of placing the glass member on a ring-shaped holding die, e.g., a ceramic block called air floating bed or the like, heating die glass member to the glass softening temperature in a furnace, and pressing the glass member with press dies to bend and form the glass member. Since the glass member originally at ambient temperature is heated and softened in this method, the energy efficiency is lower than that generally attained in the direct press method. However, the glass does not need to be softened to low viscosity (that is, the glass can be press-molded at a low temperature) in the direct press method. Therefore, the thermal contraction after molding while cooling down to ambient temperature is smaller compared to the direct press method, thereby enabling to obtain a molded article with high-precision dimensions.
When a typical reheat press method is used with a glass chunk having a small surface area to create a glass member with a shape having a large surface area relative to its volume, the glass member's temperature drops due to rapid heat exchange with the press dies as the surface area of the glass member increases during its molding process; thus, its viscosity quickly increases due to the temperature drop. Accordingly, the viscosity may inevitably increase to a degree that plastic deformation becomes difficult before the predetermined shape is achieved. That is, there is a limit to moldability. A similar problem occurs for the case of using the direct press method as well.
Thus, in the case where it is impossible to mold a glass member with a large surface area relative to its volume by using the conventional reheat press method, in particular, in the case where it is desirable to mold a thin plate-like glass member with a uniform thickness, two molding methods are typically employed: the bending and forming method, and the thin plate press molding method.
Bending and Forming Method:
In the bending and forming method, a flat glass plate that is sufficiently smooth is heated and softened while its lower surface is in contact with a die (also called a mold) to bend and form the glass plate conformal to the die surface shape by relying on the glass plate's own weight, or using vacuum suction force and the like, in the case of using the glass plate's own weight, the glass plate is placed on top of a die that was pre-formed to the glass plate's final shape, and the glass plate is heated to a high enough temperature to deform with its own weight to attain a predetermined bent shape.
In order to prevent the glass plate from deforming away from the predetermined shape after releasing from the die, the molded article is first cooled on the die to a temperature lower than the glass plate's strain point, and then, released from the die. In this case, the glass plate and the die are in contact for an extended period of time; in order to prevent any die marks on the glass plate surface in contact with the die, there is disclosed a method including interposing a heat-resistant sheet between the die and the glass plate (see JP P2012-116722A).
In the bending and forming method, a die material is not required to be very strong since the difference in temperature between the glass and the die is not large, and since the stress applied to the die is not large. Rather, inexpensive and easy-to-process ceramics with sufficient fire resistance, such as diatomite, are often used in multiple for achieving high productivity.
Thin Plate Press Molding Method:
The thin plate press molding method is a reheat press method used for thin plate-like glass members. Similar to the aforementioned bending and forming method, in the thin plate press molding method, a flat glass plate is used, which is sufficiently smooth and has the same thickness as the final product. The glass plate is heated until it has a predetermined viscosity, and then is pressed by applying a pressure with press dies. In many instances, in order to mitigate the worsening of surface roughness, the glass plate is heated to a predetermined temperature, softened and processed with weight bending while being placed on a heat-resistant fiber sheet. Thereafter, press-molding is carried out with the heat-resistant fiber sheet interposed between the upper press die and the glass plate (see JP A2003-160346).
In order to see high-resolution characters and images on a small display of a smart phone, for example, cover glass with no distortion and an extremely smooth surface is required. Considerations for future products include, for example, curving the surface of smartphone display in order to make it easier to see and handle, or deeply folding the cover glass periphery in order to increase the effective area of the display. Similarly, concave mirrors for magnified projection for vehicle-mounted head-up displays are required to be molded into an accurate shape in compliance with design parameter values, and to have an extremely smooth surface for accurately projecting images.
Shape precision of concave mirrors for head-up displays or smartphone cover glass is represented by a PV value. The PV value is a maximum shape error with respect to a design parameter value for the curved shape of a molded, bent glass plate, and is expressed based on a difference between the highest point (peak) and the lowest point (valley) in the measured range. It is required to be 50 μm or less. When the PV value exceeds 50 μm, images viewed through the cover glass or straight lines in characters and images projected on the windshield become distorted due to the excess curvature.
As for surface roughness, the surface irregularity is represented by the arithmetic average of roughness, Ra (as stipulated by JIS B0601). Smoothness of Ra≤5 nm is required. The Ra value exceeding 5 nm corresponds to the irregularity exceeding 10 nm. In this case, corner portions of characters and images become blurry and unclear; hence, the image resolution deteriorates.
Although smartphones with curved cover glass have been put into practical applications to some extent, the technology is not optimized and the research and development are still in progress to achieve precision molding for fabricating such smartphone cover glass, which can meet the requirements for the aforementioned shape precision and surface smoothness. As for a head-up display, the size of concave mirrors needs to be increased while maintaining shape precision and smoothness in order to display images on the entire automobile windshield. A suitable technology has yet to be identified at present.
In the bending and forming method, the upper surface of the glass plate does not contact with another thing, so that the glass plate member can be molded with the smoothness of at least its upper surface uncompromised. However, if there is any portion with a small curvature radius (e.g., the portion being deeply bent) required in the molded article, it becomes difficult to achieve the curvature solely by the glass plate's own weight, and thus the glass surface will not fully take the die shape. When such deep bending is required, a vacuum suction force, for example, is needed to bend the glass plate so that its lower surface will take the die surface shape. It is still difficult, however, to configure a die structure and a bending method that enables the application, of vacuum suction force over the entire glass plate surface. Furthermore, it is still difficult to mold the glass plate into its intended shape using only the glass self-weight and/or the vacuum suction force.
In the bending and forming method, the die contacting the lower surface of the glass plate member needs to be heated to a temperature near the softening point of the glass plate, and further after the molding, the die needs to be cooled to a temperature lower than, the strain point with the glass plate placed thereon to suppress the deformation. Thus, each cycle takes a long time, making it difficult to ensure sufficient productivity unless many molding dies are used at a time.
On the other hand, in the conventional thin plate press molding method discussed above, it should be possible to achieve even a small curvature radius of the glass plate since the glass plate heated to a temperature that enables plastic deformation can be formed into a predetermined shape defined by a pair of upper and lower dies. Unlike the bending and forming method, in the conventional thin plate press molding method, both the upper and lower surfaces of the glass plate will contact with the press dies under a predetermined press pressure. As a result, it is difficult to prevent the press dies' surface roughness from being substantially transferred to the molded glass article unless a heat-resistant fiber sheet or the like is interposed.
When a heat-resistant fiber sheet is interposed for reducing the surface roughness, the heat conduction between the press dies and the glass plate will be substantially compromised. Thus, in order for the temperatures of the press dies and the glass plate to be substantially equal, the press dies and the glass plate need to be kept in a pressed state for an extended period of time. With this restriction, as in the case of using the bending and forming method, each cycle takes a long time, and sufficient productivity cannot be ensured with a small number of press dies.
It may be contemplated to reduce the surface roughness of the press die to reduce the surface roughness of the bent glass plate. However, it is considered to be practically impossible to polish the press die surface to the surface roughness of 10 nm or less, and therefore, it is difficult to achieve a high smoothness of 10 nm or less for bent glass plates by only reducing the press die surface roughness.